1. Field of the Invention
The invention relates to a brake system for motor vehicles, with a master cylinder to which wheel brake cylinders are connectable, with a first piston which is coupled to a brake pedal via a push rod transmitting actuating forces, with a second piston which actuates the master cylinder, with a third piston which can be actuated by the first piston and which can be brought into force-transmitting connection to the second piston, with at least one elastic element which forms a pedal travel simulator which, in the “brake-by-wire” operating mode, gives the vehicle driver a pleasant pedal sensation, with means for coupling the movements of the first and of the third piston, with an interspace, capable of being acted upon with hydraulic pressure, between the second and the third piston, the action of pressure upon the interspace loading the second and the third piston in opposite directions, with an electrically controllable pressure supply module which allows both a filling of the interspace with pressure medium and the emptying thereof, and with a valve device, by means of which the pressure fed in the interspace can be varied.
2. Description of the Related Art
In motor vehicle technology, “brake-by-wire” brake systems are in increasingly widespread use. In these brake systems, the brake can be “extraneously” actuated, even without the active assistance of the driver, by means of electronic signals. These electronic signals may be output, for example, by an electronic stability program ESP or a headway control system ACC. If there is an overlap of an extraneous actuation of this type with a driver actuation, the driver of the motor vehicle detects a reaction in the brake pedal. This reaction effect on the brake pedal may be surprising and unpleasant to the driver, and therefore, in a critical road traffic situation, the driver does not actuate the brake pedal to an extent matched to this situation, since he is irritated by the reaction on the brake pedal caused by the extraneous actuation of the brake.
A brake system of the generic type initially mentioned is known from international patent application WO 2006/042823 A1. In order to make it possible to assist the actuating force even in the event of a failure of the control electronics or of the electrical energy supply, the known brake system has arranged between the first and the third piston a fourth piston which, in interaction with the third piston, delimits a hydraulic chamber which has a hydraulic connection, capable of being shut off as a result of brake pedal actuation, to a pressure medium reservoir, and in which pressure can be built up by means of the actuating forces, a valve arrangement being activatable by the hydraulic pressure fed in the chamber, and the pressure fed in the interspace being variable by means of said valve arrangement. It is found to be a disadvantage of the known brake system that the pedal travel simulator is switched off as a function of the movement travel of the third piston which constitutes a loss of travel during actuation.